1. Field of the Invention
The subject matter of the invention is a device for converting exhaust gas constituents of an internal combustion engine by means of at least one catalyzer and/or particle filter and/or particle separator and for compensating relative movements between the internal combustion engine and the exhaust gas train and/or relative movements between different parts of the exhaust gas train by means of at least one compensator permitting relative movements.
2. Description of the Related Art
Due to increasingly stricter limits on exhaust gas which can no longer be met entirely through measures undertaken in the engine, most internal combustion engines have been outfitted in the meantime with aftertreatment systems for reducing harmful emissions.
Examples of such aftertreatment systems include                three-way catalyzers        NOX storage catalyzers        diesel oxidation catalyzers        SCR catalyzers        particle filters.        
Since particle filters and SCR catalyzers represent a relatively new development, they will be explained briefly in the following.
Along with solids particles, nitrogen oxides are some of the limited components of exhaust gas which are formed during combustion processes. Permissible emissions of these components continue to be lowered. At present, various methods are employed to minimize these exhaust gas components in internal combustion engines for motor vehicles. Reduction of nitrogen oxides is usually accomplished by means of catalyzers; reductants are additionally required in oxygen-rich exhaust to increase selectivity and NOX conversion. These methods have come to be known under the umbrella term of SCR (Selective Catalytic Reduction) methods. They have been used for many years in the energy industry and more recently also in internal combustion engines. A detailed exposition of these methods is given in DE 34 28 232 A1. V2O5-containing mixed oxides, e.g., in the form of V2O5/WO3/TiO2, can be used as SCR catalysts. V2O5 proportions typically range between 0.2% and 3%. In practical applications, ammonia or compounds which split off ammonia such as urea or ammonia formiate are used in solid form or in solution as reductants. One mole of ammonia is needed to convert one mole of nitrogen monoxide.4NO+4NH3+O2 4N2+6H2O  (1)
When a platinum-containing NO oxidation catalyzer for forming NO2 is positioned in front of the SCR catalyzers2NO+O2 2NO2  (2)the SCR reaction is accelerated considerably and the low-temperature activity is markedly increased.NO+2NH3+NO2 2N2+3H2O  (3)
Nitrogen oxide reduction using the SCR method in internal combustion engines operating in vehicles is difficult because of the changing operating conditions, which makes it difficult to apportion the reductant in terms of quantity. On the one hand, the greatest possible conversion of nitrogen oxides must be achieved; but on the other hand emission of unspent ammonia must be prevented. This problem is often solved by using an ammonia blocking catalyzer downstream of the SCR catalyzer to convert the excess ammonia to nitrogen and water vapor. Further, the use of V2O5 as active material for the SCR catalyzer leads to problems when the exhaust gas temperature at the SCR catalyzer exceeds 650° C. because the V2O5 is sublimated. For this reason, V2O5-free iron zeolites or copper zeolites are used for high-temperature applications. Particle separators, as they are called, or particle filters are used in power plants and vehicles to minimize fine particles. A typical arrangement with particle separators for use in vehicles is described, for example, in EP 1 072 765 A1. Arrangements of this kind differ from those using particle filters in that the diameter of the channels in the particle separator is substantially greater than the diameter of the largest occurring particle, while the diameter of the filter channels in particle filters is in the range of the diameter of the particles. Due to this difference, particle filters are subject to clogging, which increases the exhaust gas back pressure and lowers engine performance. An arrangement and a method with particle filters are shown in U.S. Pat. No. 4,902,487. A distinguishing feature of the two above-mentioned arrangements and methods is that the oxidation catalyzer—usually a catalyzer with platinum as active material—arranged upstream of the particle separator or particle filter oxidizes the nitrogen monoxide in the exhaust gas by means of the residual oxygen that is also contained to form nitrogen dioxide which is converted in turn in the particle separator or particle filter with the carbon particles to form CO, CO2, N2 and NO. In this way, a continuous removal of the deposited solids particles is carried out.2NO2+C 2NO+CO2  (4)NO2+C NO+CO  (5)2C+2NO2 N2+2CO2  (6)
Another possibility for removing the soot particles deposited in the particle separator or particle filter is to oxidize these soot particles in regeneration cycles at high temperatures with the oxygen present in the exhaust gas flow.C+O2 CO2  (7)
In order to meet future exhaust gas regulations, it will be necessary to use arrangements for reducing nitrogen oxide emissions and arrangements for reducing fine particles emissions at the same time. Various arrangements and methods are already known for this purpose.
DE 103 48 799 A1 describes an arrangement comprising an oxidation catalyzer, a SCR catalyzer arranged downstream of the latter in the exhaust gas flow, and a particle filter which is again arranged downstream in the exhaust gas flow. The reductant for the selective catalytic reaction taking place in the SCR catalyzer is fed in immediately in front of the SCR catalyzer by a urea injection device that is controlled as a function of the operating parameters of the internal combustion engine. A disadvantage in this arrangement is that the nitrogen dioxide generated in the oxidation catalyzer is substantially completely consumed by the selective catalytic reduction in the SCR catalyzer; that is, it is no longer available for the conversion of the solids particles that have accumulated in the particle filter arranged downstream. Therefore, the regeneration of the particle filter must be carried out uneconomically through cyclical heating of the exhaust gas flow by enriching the exhaust gas flow with unconsumed hydrocarbons. This is accomplished either by enriching the combustion mixture or by injecting fuel in front of the particle filter. On the one hand, an arrangement of this kind for regenerating the particle filter is elaborate and therefore expensive. On the other hand, the cyclical regeneration of the particle filter situated at the end of the arrangement produces harmful substances again which can no longer be removed from the exhaust gas. Further, when particle filters are used, the filters can be clogged by engine oil ashes so that these filters must routinely be removed and cleaned.
U.S. Pat. No. 6,805,849 discloses another combination of a particle filter and an arrangement for selective catalytic reduction. The arrangement described therein includes an oxidation catalyzer which is arranged in the exhaust gas flow and which increases the proportion of nitrogen dioxide in the exhaust gas, a solids filter arranged downstream, a reservoir for the reducing liquid, an injection device for the reducing liquid which is arranged behind the solids filter, and a SCR catalyzer which is arranged downstream of the latter in the exhaust gas flow. While the arrangement described above allows a continuous conversion of the soot-type solid particles deposited in the solids filter by means of the nitrogen dioxide generated in the oxidation catalyzer, it has a serious drawback. The particle filter causes a cooling of the exhaust gas so that, for example, when using the reducing liquid known as AdBlue which is presently commercially available, the exhaust gas temperature, particularly after the internal combustion engine is started or when the internal combustion engine is operated in the lower output range, is too low to generate ammonia from the 33-% aqueous urea solution without the occurrence of problematic byproducts.
In connection with the decomposition of urea ((NH2)2CO) in ammonia (NH3), it is known that this takes place under optimal conditions (temperatures above 350° C.) in two steps. First, thermolysis, i.e., the thermal decomposition, of urea takes place according to the following reaction:(NH2)2CO NH3+HNCO  (8)
This is followed by hydrolysis, that is, the catalytic decomposition, of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2) according to the following reaction:HNCP+H2O NH3+CO2  (9)
Due to the fact that the reductant is in the form of an aqueous solution when AdBlue is used, this water must evaporate before and during the actual thermolysis and hydrolysis.
If the temperatures during the above-mentioned reactions (8) and (9) are below 350° C. or if heating is only gradual, it is known from DE 40 38 054 A1 that chiefly solid, infusible cyanuric acid is formed through trimerization of the isocyanic acid formed in (8):3NHCO<350° C.→←>350° C.(HNCO)3  (10)leading to clogging of the SCR catalyzer downstream. As is stated in DE 40 38 054, cited above, this problem can be remedied in that the exhaust gas flow charged with the reductant is guided through a hydrolysis catalyzer. Thus, the exhaust gas temperature at which a quantitative hydrolysis is first possible can be brought down to 160° C. The construction and composition of a corresponding catalyzer is likewise described in the above-cited publication as is the construction and operation of a SCR catalyzer system outfitted with a hydrolysis catalyzer.
In order to reduce the size of the catalyzers while maintaining a constant dwell time in the catalyzers, the hydrolysis catalyzers are also operated in a partial flow of exhaust gas that is removed from the exhaust gas flow and then fed back into it after hydrolysis. A corresponding arrangement is shown in EP 1052009 A1. In this connection, it is particularly advantageous when the partial exhaust gas flow is taken off as close as possible to the engine so that the hydrolysis catalyzer can be operated at a high temperature level. Further, in internal combustion engines charged with exhaust gas it is advantageous to remove the partial exhaust gas flow already before the turbocharger and to return it downstream of the turbocharger. However, removing the partial exhaust gas flow close to the engine and metering the reductant leads to a problem. In certain operating states of the internal combustion engine, chiefly in low-load operation, overrun operation, engine braking operation, in idling phases or when turning off the engine, a reversal of the flow direction of the exhaust gas can occur so that reductant, ammonia split off from the reductant, or byproducts formed from the reductant, e.g., isocyanic acid (Equation 9), cyanuric acid (Equation 10), and so on, can come into contact with the parts of the engine in contact with the exhaust gas due to return flows and/or diffusion in direction of the engine block. This can lead to corrosion of the materials installed in the engine block, particularly the seals.
One solution to this problem would be to arrange a catalyzer having an oxidizing activity on the decomposition products of the urea between the end position and the engine block, where these highly corrosive compounds would be destroyed in case of a return flow.
However, an additional catalyzer would exacerbate the problem of space which already exists due to the number of catalyzers that are used, the reductant injection system and the particle separators, because there is often insufficient installation space available particularly in internal combustion engines installed in vehicles.
In connection with the exhaust gas train in internal combustion engines, that is, the part which also holds the catalyzers, it is known from U.S. Pat. No. 6,610,506 to install compensators. These are flexible structural component parts which compensate for the mechanical vibrations of the engine and for the thermal expansions of the exhaust gas train. They are integrated in the exhaust gas train and, therefore, exhaust gas flows through them.